Along with the advent of information era, a broadband capable of communicating a large volume of information at a high speed becomes widespread. Transmission devices such as a router device and a WDM (Wavelength Division Multiplexing) device are used as devices for sending information to the broadband.
Optical communication technology for transferring data through use of optical carrier waves is employed in the transmission devices. A multiplicity of optical waveguides are used as a means for guiding the optical carrier waves from one point to another.
Each of the optical waveguides includes a linear core portion and a cladding portion configured so as to cover the core portion. The core portion is made of a material which does not substantially absorb the optical carrier waves, while the cladding portion is formed of a material having a refractive index smaller than that of the core portion.
In the optical waveguide having such a configuration, light introduced from one end of the core portion is reflected by an interfacial surface between the core portion and the cladding portion and then transferred to the other end of the core portion. A light emitting element such as a semiconductor laser is arranged at an incoming side of the optical waveguide.
On the other hand, a light receiving element such as a photodiode is arranged at an outgoing side of the optical waveguide. Light emitted from the light emitting element travels along the optical waveguide and arrives at the light receiving element. Communication is performed according to flickering patterns of the light received by the light receiving element.
In recent years, a quantity of information treated by the transmission devices tends to increase, thereby requiring an increased transmission speed and an increased density of the optical waveguide. There are also many demands for size reduction and density increase of the transmission devices. In other words, a demand has existed for a multichannel type light transmission module having reduced light loss and increased reliability.
For the above demand, patent document 1 discloses a laminated type flexible optical waveguide having a plurality of core portions (waveguide cores) and exhibiting high bendability. Such an optical waveguide is a multichannel type and is therefore capable of transmitting a large quantity of information. Further, the optical waveguide is made of a transparent resin material and can make communication even when being bent. This makes it possible to efficiently use a mounting space within a transmission device.
Further, patent document 2 discloses a prior art regarding an optical waveguide having a film shape similar to that disclosed in patent document 1.
The conventional optical waveguide is usually made of a resin material. Therefore, when being solidified in a production process of the optical waveguide, the resin material is shrunken. This means that shrinkage of the optical waveguide is unavoidable. As a consequence, dimensional accuracy between the core portions (channels) of the optical waveguide is reduced, which in turn impairs connectivity of the optical waveguide with a connection counterpart (e.g., connector).
In other words, a size of a gap between the core portions of the optical waveguide, through which light is propagated, is changed due to the shrinkage of the optical waveguide. Thus, a channel pitch of the optical waveguide does not match a channel pitch of the connector. This increases optical connection loss in an optical connection portion between the optical waveguide and the connector, which may possibly impair the quality of optical communication.
If the conventional multichannel type optical waveguide is optically connected to a multi-core connector such as a MPO connector (JIS C 5982), there is posed a problem in that the optical connection loss becomes greater. As described above, the optical waveguide is molded with a polymer material. Inasmuch as the optical waveguide is shrunken in a molding process thereof, a pitch (gap) between the core portions of the optical waveguide is also reduced in the molding process thereof.
Difficulties are involved in accurately controlling and precisely estimating a shrinkage ratio of the optical waveguide. For this reason, when the multichannel type optical waveguide is connected to the multi-core optical connector, positional misalignment occurs between optical fiber cores of the multi-core optical connector and the core portions of the optical waveguide. Presumably, this increases the optical connection loss.
FIG. 33 is a view schematically showing a positional relationship between core portions 99 of a conventional multichannel type optical waveguide 990 and optical fiber cores 810 and 820 of multi-core optical connectors 81 and 82 as connection counterparts.
As shown in FIG. 33, the multichannel type optical waveguide 990 is usually connected to the multi-core optical connectors 81 and 82 provided with the optical fiber cores 810 and 820 corresponding to an arrangement of the core portions 99. This makes it possible to propagate optical signals between the optical waveguide 990 and the multi-core optical connectors 81 and 82.
If a resin material is heavily shrunken in a production process of the optical waveguide 990, however, positional misalignment occurs between optical axes of the core portions 99 of the optical waveguide 990 and optical axes of the optical fiber cores 810 and 820 of the multi-core optical connectors 81 and 82 as shown in FIG. 33. This increases the optical connection loss between the multi-core optical connectors 81 and 82 and the optical waveguide 990.
In addition, there is a possibility that an optical signal propagating through one of the core portions (channels) 99 is not received to a right optical fiber core (channel) 820, but is leaked to an adjoining optical fiber core 820 in the connection portion to the multi-core optical connector 82.
On the contrary, there is also a possibility that an optical signal propagating through one of the optical fiber cores 810 is not received to a right core portion 99, but is leaked to an adjoining core portion 99 in the connection portion to the multi-core optical connector 81 (which is called “cross torque”).
Occurrence of such leakage impairs the quality of optical communication.
It is difficult to accurately control a shrinkage ratio of the resin material. Even if the optical waveguide is designed by preliminarily assuming the shrinkage of the resin material, dimensional accuracy of the optical waveguide becomes insufficient.
On the other hand, the above optical waveguide also can be obtained by dividing an optical waveguide film. Specifically, the optical waveguide film is cut along longitudinal directions of cladding portions included therein so that the optical waveguide film is divided into a plurality of strip-shaped optical waveguides.
In this cutting process, the plurality of cladding portions can be cut at one time through use of a multi-blade saw having a plurality of saw blades arranged at an equal interval. At this time, there is a need to match a size of a gap between the core portions of the optical waveguide film with a size of a gap between the saw blades.
FIGS. 34(a) and 34(b) are views each explaining a method of cutting a conventional multichannel type optical waveguide film 10′ using a multi-blade saw. FIG. 34(a) is a front view of the optical waveguide film 10′ as seen from an end portion side thereof and FIG. 34(b) is a top view of the optical waveguide film 10′.
The optical waveguide film 10′ is formed of a laminated body in which a cladding layer 901, a core layer 903 and a cladding layer 902 are laminated one above another. The core layer 903 includes a plurality of rectilinear core portions 904 arranged side by side at an equal interval when seen in a plan view and a plurality of cladding portions 905 adjoining to the respective core portions 904.
As shown in FIG. 34(a), a multi-blade saw 7 having a plurality of saw blades 71 arranged at an equal interval is positioned above the optical waveguide film 10′. Each of the saw blades 71 has a circular shape when seen in a plan view. A rotation shaft 72 passes through central portions of the saw blades 71.
In such a multi-blade saw 7, the saw blades 71 are pressed against the optical waveguide film 10′ while rotating the rotation shaft 72. A size of a gap between the saw blades 71 is preliminarily adjusted in conformity with a size of a gap (pitch) between the cladding portions 905. Thus, the optical waveguide film 10′ is cut in positions corresponding to central lines of the respective cladding portions 905.
The optical waveguide film 10′ is severed into a plurality of strip-shaped optical waveguides 90 at one time by moving the saw blades 71 along longitudinal directions of the respective cladding portions 905 of the optical waveguide film 10′ as indicated by arrows in FIG. 34(b).
In the meantime, a resin material as a raw material of the optical waveguide film 10′ is subjected to shrinkage as it is solidified in a production process thereof. Since a shrinkage ratio of the resin material is affected by factors such as a composition or raw substances of the resin material and a production environment, dimension of the optical waveguide film 10′ necessarily involves individual variability.
In the case where the optical waveguide film 10′ having such individual dimension variability is cut with the blade saw 7, the size of the gap between the saw blades 71 need to be adjusted on a case-by-case basis in conformity with the individual dimension variability. As a result, efficiency of cutting works is reduced sharply.
Further, if a plurality of optical waveguide films 10′ are cut with no adjustment of the size of the gap between the saw blades 71, positions of the core portions 904 in each of the optical waveguide films 10′ are deviated according to the individual dimension variability.
Consequently, the core portions 904 are off-centered in each of the optical waveguides 90 thus diced, which impairs connectivity of each of the optical waveguides 90. In other words, the off-centering of the core portions 904 increases light loss in a connection portion between each of the optical waveguides 90 and a connection counterpart thereof.
Further, off-centering amounts of the core portions 904 tend to be cumulatively increased toward outer sides of the optical waveguide film 10′. Therefore, in each of the optical waveguides 90 diced from outer sides of the optical waveguide film 10′, there is a case that cutting traces of the saw blades 71 interfere with a part of the core portions 904. Such optical waveguides may possibly lower their functions.    Patent document 1: JP-A 2007-84765    Patent document 2: JP-A 2006-23385